1. Field of the Invention
This invention relates generally to the vapor composition in a reactor used for carrying out a liquid phase reaction involving molecular oxygen. In particular, the invention relates to minimizing the inert gas flow supplied to a liquid phase oxidation reactor in order to prevent combustion, deflagration, or detonation in the liquid phase oxidation reactor headspace.
2. Related Background Art
Air-based, as well as oxygen-based, oxidation reactions involving flammable liquids require that the vapor or gas space above the liquid (the reactor headspace) be maintained outside the flammability range. Air-based reactions utilize molecular oxygen from an atmospheric composition, by volume percent, of gases. The atmospheric composition (air) may be at a different pressure than atmospheric pressure. Oxygen-based reactions utilize molecular oxygen from a substantially pure oxygen gas.
In oxygen-based liquid phase oxidation reactions, supplemental nitrogen is typically fed to the headspace in order to maintain the concentration of its components outside the flammability limits; in the absence of nitrogen and if the vapor pressure of the flammable liquid is in the proper range, it would be possible for vapor bubbles to enter the reactor headspace having a flammable composition.
In air-based liquid phase oxidation processes the reactor is typically designed to consume enough gaseous oxygen so that the remaining oxygen concentration is below the limiting oxygen value (LOV) for flammability. Ballast gases other than nitrogen have been described for oxidation reactions that are carried out in the gas phase. U.S. Pat. No. 3,855,280, for example, describes using methane or ethane diluents in the gas phase oxidation of ethylene and acetic acid gas to prepare vinyl acetate. European Patent No. 361,372 describes using alkanes, carbon dioxide, steam or other diluents in the gas phase production of methacrolein and methacrylic acid. U.S. Pat. No. 3,119,837 describes using methane, nitrogen, carbon dioxide, or steam in the gas phase oxidation of ethylene to form ethylene oxide.
These teachings from gas phase oxidation processes, however, cannot be extended to liquid phase oxidation processes because the liquid phase reactions present significant differences with respect to their oxidation reactions carried out in the gas phase.
One difference is related to the fact that liquid phase oxidation processes typically involve a step of recondensing a process liquid such as the reaction solvent. Accordingly, if a ballast gas other than nitrogen is used in the liquid phase process, the effects of the condensation step must be considered. Another difference is that, in contrast to gas phase oxidation processes, no heat transfer benefit is realized in the liquid phase oxidation reaction with respect to the choice of a particular inerting gas.
While the flammability of the gas phase reaction mixture depends essentially on its components, flammability in the liquid phase reactor is typically a function of temperature, pressure and on the physical properties of the flammable process liquid, often the solvent.
Furthermore, whereas in a gas phase oxidation reactor it must be ensured that the reactor composition is outside the flammability regime at every point in the reactor, only the reactor headspace raises similar concerns for a liquid phase oxidation process.
U.S. Pat. No. 5,430,181 mentions the possibility of adding a third gas such as methane, ethane or helium to liquid phase oxidations but does not provide any detail on how this may be accomplished.
U.S. Pat. No. 5,693,856 describes the use of recycled carbon dioxide as a flammability suppressor in a terephthalic acid producing reactor. No mention is made with regard to the amount of CO.sub.2 to be added to the reactor and, compared to using nitrogen, the described process does not reduce the flow of inerting gas.
U.S. Pat. No. 5,552,131 describes a highly specialized reactor design and an elaborate way of adding O.sub.2 to the reactor liquid phase for a particular reaction. A ballast gas (methane, ethane and propane) is added to the product stream to avoid flammability problems. Again, the amount of the ballast gas to be added is not discussed.
One problem not addressed by the prior art is the fact that any supplemental inerting gas added to the liquid phase process must be treated with the off-gas. Typically, the use of such additional inerting gas disadvantageously raises the off-gas treatment costs and increases the required equipment sizes. Thus, it would be desirable to provide additional inerting gas without raising off-gas treatment costs.
Specifically, in order to handle the additional gas, reactor condensers, separation equipment, and vent gas treatment equipment must be larger. Process utilities such as cooling water, steam, electricity, and fuel for the incineration of waste gases are also increased when supplemental inert gas is used.
Accordingly, it is an object of this invention to provide a method of reducing the amount of inert gas required to render the liquid phase reaction reactor headspace nonflammable.